1. Field of the Invention
The invention relates to an electrostatic discharge protective circuit, and in particular to an electrostatic discharge protective circuit for protecting an internal circuit against damage from an electrostatic discharge by using a silicon controller rectifier (SCR).
2. Description of the Prior Art
Currently, developments of semiconductor technologies for IC manufacturing have been continuous towards miniaturization and higher integration density. As a result. ICs constituted especially by metal oxide semiconductor (MOS) transistors are more easily damaged by an electrostatic discharge (hereinafter referred to as ESD). In practice, an ESD could release several hundreds of volts or higher to cause damage on the internal circuits of ICs. Many factors could cause the ESD. One of the factors is that electrostatic charges are continuously accumulated. Therefore, how to timely release the accumulated electrostatic charges becomes a very important issue. To handle with this issue, a common prior method uses a silicon controlled rectifier (SCR) to realize the ESD protection. The detailed description of the Prior method will be given hereinafter.
Referring to FIG. 1A, an SCR, functioning an electrostatic discharge protective circuit, for an ESD protection according to the prior art is shown. In FIG. 1A, the emitter of a transistor 110 is electrically connected to an anode while the collector thereof is electrically coupled to a cathode via a resistor R.sub.2. The collector of a transistor 120 is electrically coupled to the anode via a resistor R.sub.2 while the emitter thereof is directly electrically connected to the cathode. Furthermore, the collector of the transistor 110 is electrically coupled to base of the transistor 120. The collector of the transistor 120 is electrically coupled to the base of the transistor 110.
A bonding pad of an IC (not shown) required to be protected against damage from an electrostatic discharge is electrically connected to the anode. Thus, electrostatic charges accumulated on the bonding pad can be bypassed via the SCR. For instant, when electrostatic charges are increasingly accumulated on the bonding pad to cause a breakdown on the transistor 110, a first current is generated and flows from the anode via the emitter and collector of the transistor 110 and the resistor R1 to the cathode. At this time, a voltage drop is created on both ends of the resistor R1 to turn on the transistor 120 due to the first current flowing through the resistor R.sub.1. After the transistor 120 is turned on and operates within an active region, a second current is generated and flows from the anode via the resistor R.sub.2, and the collector and emitter of the transistor 120 to the cathode. Moreover, the transistor 110 call be provided with a bias voltage created by the second current flowing through the resistor R.sub.2, thereby allowing the transistor 110 to operate within an active region. Since the transistors 110 and 120 all operate within an active region, the accumulated electrostatic charges can be smoothly discharged so as to lower the voltage of the bonding pad. When the voltage of the anode (bonding pad) is insufficient to maintain the transistors 110 and 120 to operate within an active region, the transistors 110 and 120 are turned off.
Referring to FIG. 1B, there is shown an I-V cure of the SCR of FIG. 1A. In FIG. 1B, when the potential of the anode is increased to the breakdown voltage V.sub.t of the transistor 110, the transistor 110 is turned on to provide a required bias voltage to the transistor 120, thereby turning on the transistor 120. Then, the turned-on transistor 120 provides a required bias voltage to the transistor 110 to allow both transistors 110 and 120 to operate within an active region and approximately maintain the potential of the anode at a holding voltage (V.sub.h).
As the SCR is turned on, the turned-on resistance and V.sub.h are very low. Therefore, an ESD protection can be perfectly realized. Unfortunately, a trigger current require by the SCR is also very low. Under this circumstance, noises can easily cause the SCR to be erroneously turned on.